How accurate is LMP dating?

Within about a week for regular 28-day cycles; ultrasound is more precise.

What if my cycle isn't 28 days?

Add (cycle length − 28) to the estimated due date.

Only about 4% of babies arrive on the exact due date; 37–42 weeks is normal.

Yes — prenatal care from early pregnancy is important.

Estimate your due date using Naegele's rule based on the last menstrual period (LMP).

All math runs in your browser — nothing is uploadedReviewed & updated: January 1970Methodology

First day of last period (LMP)Average cycle length (days)

Week

of 40

Estimated due dateFri Apr 23 2027

Estimated conceptionFri Jul 31 2026

Weeks along0 weeks

Due date = LMP + 280 days (40 weeks). Adjust for cycle length ≠ 28 days.

Example

LMP Jan 1 → due date ≈ Oct 8.

How accurate is LMP dating?: Within about a week for regular 28-day cycles; ultrasound is more precise.
What if my cycle isn't 28 days?: Add (cycle length − 28) to the estimated due date.
Is 40 weeks always right?: Only about 4% of babies arrive on the exact due date; 37–42 weeks is normal.
Should I see a doctor?: Yes — prenatal care from early pregnancy is important.

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